enterwrite is not OA_DANGEROUS

enterwrite is not OA_DANGEROUS

OA_DANGEROUS indicates that temporary copies may need to be made in
list assignment, to handle things like:

($a, $b) = ($b, $a);

In other words, an op type is flagged with OA_DANGEROUS if its return
values could occur elsewhere on the stack.

‘write’ only ever returns a read-only true or false, so temp copies
are not necessary for its sake.